The present invention relates to the art of polybenzazole (PBZ) polymers and polymer compositions containing blocks of those polymers.
PBZ polymers, i.e., polybenzoxazole, polybenzothiazole and polybenzimidazole, and their synthesis are described in great detail in the following patents which are incorporated by reference: Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,703,103 (Oct. 27, 1987); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,692 (Aug. 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzothiazole) Compositions, Process and Products, U.S. Pat. No. 4,533,724 (Aug. 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,693 (Aug. 6, 1985); Evers, Thermoxidatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Pat. No. 4,359,567 (Nov. 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,578,432 (Mar. 25, 1986) and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988).
Polybenzazole polymers, and particularly "rigid rod" PBZ polymers, are noted for high tensile strength, high tensile modulus and high thermal stability. However, many polybenzazole polymers are difficult to fabricate into useful articles. Rigid and semi-rigid polybenzazoles do not have glass transition temperatures at any temperature at which they are stable. Therefore, the polymers are ordinarily spun from solution to form fibers, which serve as reinforcement within a thermosetting matrix, such as epoxy resins, to form composites. However, the fibers and the cured composites are not moldable or thermoformable.
Many moldable and thermoformable polymers are known. Exemplary polymers include thermoplastic polyamides, polyimides, polyquinolines, polyquinoxalines, poly(aromatic ether ketones) and poly(aromatic ether sulfones). However, those polymers do not have the high tensile strength and modulus which are characteristic of polybenzazole polymers.
Attempts have been made to synthesize articles which combine the processability of the thermoplastic polymer with the superior physical properties of the polybenzazole polymer. To this end, molecular composites of rigid rod polybenzazole and flexible polymers have been studied. Such molecular composites are described in numerous references, such as U.S. Pat. Nos. 4,207,407; 4,377,546; 4,631,318; 4,749,753 and 4,810,735, and Hwang et al., "Solution Processing and Properties of Molecular Composite Fibers and Films," 23 Polymer Eng. & Sci. 784 (1983); Hwang et al., "Phase Relationships of Rigid Rod Polymer/Flexible Coil Polymer/Solvent Ternary Systems," 23 Polymer Eng. & Sci. 789 (1983); and Hwang et al., "Composites on a Molecular Level: Phase Relationships, Processing and Properties," B22 J. Macromol. Sci.-Phys. 231 (1983), which are all incorporated herein by reference.
However, polybenzazole, and particularly rigid and semi-rigid polybenzazole, are incompatible with many thermoplastic polymers. When dopes containing polybenzazole and a thermoplastic polymer are coagulated, the polybenzazole agglomerates and/or phase separates. The resulting shaped articles either have poorer properties in all directions than the corresponding thermoplastic alone, or have superior properties in one direction and inferior properties in all other directions. Such compositions are useful for fibers, but not for molded articles.
What are needed are materials and processes which can be used to make molded articles containing reinforcing amounts of polybenzazole polymer which have superior properties in at least two dimensions and/or are not substantially phase separated.